potentially hazardous asteroid

Results from NASA's NEOWISE
survey find that more potentially hazardous asteroids are closely
aligned with the plane of our solar system than previous models suggested.
This diagram shows an edge-on view of our solar system. The dots represent
a snapshot of the population of NEAs and PHAs that scientists think
are likely to exist based on the NEOWISE survey. Positions of a simulated
population of PHAs on a typical day are shown in bright orange, and
the simulated NEAs are blue. Earth's orbit is green. The diagram shows
that the orbits of the PHAs tend to be more closely aligned with the
plane of our solar system, or less tilted above and below the plane,
than the NEAs. This characteristic of PHAs was known before the NEOWISE
survey. Now, NEOWISE has found the PHAs to be about twice as likely
to have these lower-inclination orbits than previously thought.

This diagram illustrates the differences between
orbits of a typical near-Earth asteroid (blue) and a potentially hazardous
asteroid, or PHA (orange). Image credit: NASA/JPL-Caltech

Scientists have adopted a very specific definition for the term "potentially
hazardous asteroid," which is any asteroid with an Earth Minimum Orbit Intersection
Distance (MOID) of 0.05 AU or less and an absolute
magnitude (H) of 22.0 or less. In other words, an asteroid that can't get
any closer to the Earth than 0.05 AU (roughly 7,480,000 kilometers or 4,650,000
miles) or is smaller than about 330 meters (500 feet) in diameter is not considered
to be a PHA. Currently just over a thousand potentially hazardous asteroids
are known, out of a total population estimated to be 4,700±1,500
(based on an analysis of data from the NEOWISE survey published in May 2012).

Measuring the risk

Planetary scientists have developed a means of conveying the risks associated
with asteroids and comets that might collide with the Earth. This risk-assessment
scale, called the Torino scale, is
similar to the Richter scale used for measuring earthquakes. The Torino
scale runs from zero to 10. An object with a value of zero or one will have
virtually no chance of causing damage on Earth; a 10 means a certain global
climatic catastrophe.

Detection and monitoring of potentially
hazardous asteroids

The growing realization that Earth will inevitably be hit hard again at
some point, with potentially catastrophic results for human and other life,
has led to the setting up of several programs to detect and monitor NEAs
(or, more generally, NEOs – near-Earth
objects – which also include short-period
comets) and near-Earth threatening objects. Among these projects are
the Spacewatch Project, LINEAR,
the Spaceguard Foundation,
and the Jet Propulsion Laboratory's NEO Program.

Recent
close approaches

Among recent close encounters was one on June 14, 2002, when an asteroid
the size of a football pitch, catalogued as 2002MN, passed Earth at a distance
of around 120,000 kilometers – less than a third the distance to the Moon
– traveling at over 10 km/s (23,000 mph). This was bettered by 1994XM1,
which came to within 105,000 kilometers of Earth in December 1994.

On Feb 15, 2013, a small asteroid exploded over Chelyabinsk, Russia, at a height of about 30km.

The threat from Apophis

The current biggest threat of a collision with Earth is posed by the asteroid Apophis. This 400-meter-wide space rock
will come closer to Earth in 2029 than the orbits of many communications
satellites – but it will not hit the planet. The concern centers on
the small chance that its orbit could be altered enough during the flyby
to put the rock on a collision path for its return in 2036. In 2006, the Planetary Society announced a $50,000 prize
to be given to the designers of the mission that would allow the asteroid's
orbit to be tracked with the most precision. The competition has support
from the US and European space agencies. It was won in February 2008 by
a team led by Atlanta-based SpaceWorks Engineering. The Foresight spacecraft,
if it goes ahead, will launch in about 2012, rendezvous with Apophis, determine
its center of mass, and take regular measurements of its position relative
to the asteroid. These measurements will be used to reduce the uncertainties
in Apophis' orbit.

TNT equivalent of asteroid impacts

Even a relatively small asteroid, with a diameter of 100 meters, could have
a devastating effect on the Earth and its life because of its high incoming
velocity. The kinetic energy (energy
of motion) of an object is given by the formula

KE = ½mv2

where m is the mass of the object and v its velocity.
If the velocity is high, then velocity squared is large and this makes the
kinetic energy large if the mass is significant.

The table below shows the equivalent in megatons of TNT of the impact of
asteroids of various masses and velocities, assuming a typical rock-like
density for an asteroid of 2.5 kg/dm3. For comparison, the yields
of the Hiroshima and Nagasaki atomic bomb explosions were approximately
15,000 tons and 21,000 tons, respectively. In other words, the smallest
impact considered in the table (that of a 100-meter-wide asteroid traveling
at 15 km/s) would be roughly 100 times more powerful than the devastating
Hiroshima-Nagasaki attacks.

TNT equivalent in megatons of asteroid impacts

diameter
(meters)

mass
(million tons)

velocity (km/s)

15

20

25

30

100

1.3

35

62

97

140

200

10

280

500

780

1,100

300

35

950

1,700

2,600

3,800

400

84

2,200

4,000

6,200

9,000

600

280

7,600

13,000

21,000

30,000

800

670

18,000

32,000

50,000

72,000

1,000

1,300

35,000

62,000

97,000

140,000

2,000

10,000

280,000

500,000

780,000

1,100,000

3,000

35,000

950,000

1,700,000

2,600,000

3,800,000

4,000

84,000

2,200,000

4,000,000

6,200,000

9,000,000

5,000

160,000

4,400,000

7,800,000

12,000,000

18,000,000

6,000

280,000

7,600,000

13,000,000

21,000,000

30,000,000

7,000

450,000

12,000,000

21,000,000

33,000,000

48,000,000

8,000

670,000

18,000,000

32,000,000

50,000,000

72,000,000

9,000

950,000

26,000,000

45,000,000

71,000,000

100,000,000

10,000

1,100,000

35,000,000

62,000,000

97,000,000

140,000,000

Has the threat of 'city-buster' sized asteroids been underestimated

The B612 Foundation, a private organization pushing for a robust defense against potentially hazardous asteroids, released a video in April 2014 summarizing data collected by a network of sensors operated by the Nuclear Test Ban Treaty Organization. Between 2000 and 2013, this network detected 26 explosions ranging in magnitude from 1 to 600 kilotons. By comparison, the atomic bomb dropped over Hiroshima was equivalent to a blast of 15 kilotons. All 26 explosions were due to asteroids entering the Earth's atmosphere. One of the largest of them was the one that exploded over Chelyabinsk Oblast, Russia, in 2013. This newly disclosed data suggests we may have significantly underestimated the threat from small asteroids capable of causing significant local destruction and loss of life.